A communication partner device of the kind specified in the second paragraph above having a transmission circuit of the kind specified in the first paragraph above has been put on the market in a number of variant designs by the applicants and the device and circuit in question are therefore known. In the known communication partner device, the transmission circuit is formed by an integrated circuit or IC, which IC also includes or forms a receiving circuit and has been put on the market by the applicants under the type number MF RC 500. The IC used as a transmission circuit has two transmit signal outputs. Connected to each of the two transmit signal outputs is a transmit signal input of a signal processing circuit. The signal processing circuit includes a filter stage that comprises inductors and capacitors and is arranged to form a so-called EMC filter that is intended to ensure the electromagnetic compatibility laid down by the authorities. Connected to the filter stage is a matching stage of the signal processing circuit that comprises a combination of series capacitors and parallel capacitors and by means of which the impedance of a transmission coil connected to the signal processing circuit can be transformed to a given desired value. Connected to the matching stage is a damping stage of the signal processing circuit that is formed by means of two resistors, the above-mentioned transmission coil being connected to the damping stage. The quality factor of the transmission coil can be reduced to a required desired value by means of the damping stage. In the known design, the signal processing circuit comprises, in essence, only inductors and capacitors and the signal processing circuit is suitable and designed/arranged for transforming resistance values, with the transformation of resistance values affecting not only resistance values proper (the real part) but also values of reactance (the imaginary part).
In the known design, the modulation stage for modulating the carrier signal has two series circuits comprising firstly a plurality of PMOS transistors connected in parallel and secondly a plurality of NMOS transistors connected in parallel, with the plurality of PMOS transistors connected in parallel in each of the two series circuits being able to be controlled, in respect of the total resistance of the main current paths of the PMOS transistors which are connected in parallel, between a plurality of different values for this resistance, whereas the plurality of NMOS transistors connected in parallel in each of the two series circuits can be controlled, in respect of the total resistance of the main current paths of the NMOS transistors which are connected in parallel, only between a conductive state of conduction and a blocked state of conduction. Amplitude modulation (ASK modulation) can be performed with the help of the modulation stage to obtain a modulated carrier signal having high-level carrier signal sections and low-level carrier signal sections. To generate the high-level carrier signal sections, what is done in cyclic succession is that, during a first half-cycle of the carrier signal, all the PMOS transistors are controlled to their conductive state of conduction and at the same time all the NMOS transistors are controlled to their blocked state of conduction and, during a second half-cycle of the carrier signal, all the PMOS transistors are controlled to their blocked state of conduction and at the same time all the NMOS transistors are controlled to their conductive state of conduction. To generate the low-level carrier signal sections in the case of 100% ASK modulation, all the PMOS transistors are controlled to their blocked state of conduction and at the same time all the NMOS transistors are controlled to their conductive state of conduction for the entire length of each low-level carrier signal section. In the last-mentioned case, i.e. when the low-level carrier signal sections are generated, the very low level of resistance that is produced by means of the conductive NMOS transistors and that acts on the signal outputs is transformed, by the signal processing circuit arranged to transform resistance values, to a relatively high resistance value at the transmission coil. However, where a fast drop of the voltage present at the transmission coil is required to allow as high as possible a data transmission rate to be achieved, such a high resistance value at the transmission coil opposes and is a hindrance to any such fast drop of the voltage present at the transmission coil, which means that with a transmission coil having a high quality factor of the kind required to achieve a relatively low data transmission rate of, for example, 106 kbaud, transmission of data at an appreciably higher data transmission rate of, for example, 848 kbaud is not possible. Also, in the known design, the ratio in which the signal processing circuit transforms resistance depends on the values of components, i.e. the values of the components of the signal processing circuit, namely the components of the filter stage and the matching stage, which means that, depending on the existing values of the inductances of the inductors and of the capacitances of the capacitors, which values are subject to production-related scatter, the ratios that occur for the transformation of resistance differ. The result of this is that different transformed resistance values at the transmission coil are possible, and do in fact occur, in different communication partner devices, and as a result different damping ratios relating to the damping of the particular transmission coil exist in different communication partner devices, which is a disadvantage from the point of view of a fast change, and in particular a fast drop, in amplitude levels in the amplitude-modulated transmit signal fed to the transmission coil, and from the point of view of a fast change which is as nearly the same as possible in different communication partner devices.